Nominal refers to a thing's name, rather than its actual status in the present moment. The point of the term is to keep new folk from thinking that 12v literally means the system or device is running at 12.0 volts. It's usually not.
The most common uses of nominal for 'dwellers are:
The most common battery bank voltage for 'dwellers is 12v, although some run 24v or specialized, exotic voltages like 48v.1)
Nominal 12v banks are rarely actually 12.0v. Examples:
Similarly, “12v” solar panels are called that because they are used to charge/power 12v systems. The voltages on these vary even more widely: